1. Field of the Invention
The present invention relates to an apparatus used in nuclear magnetic resonance (NMR) imaging, and more particularly, to a new and improved toroid cavity detector that reduces the effects of probe ringing when the toroid cavity detector is used in connection with NMR spectroscopy and tomographic imaging.
2. Background of the Invention
Large inventories of uncharacterized and heterogeneous nuclear material need to be characterized. Nuclear magnetic resonance (NMR) techniques have been used as a nondestructive assay (NDA) to characterize these materials before packaging (in particular to determine moisture content) and also to examine the materials after they have been packaged. One NMR technique that has been used involves a toroid cavity probe. This technique enables the determination of both the presence and quantity of individual elements as well as full chemical-shift resolution (i.e., chemical speciation). For example, determinations can be made as to the presence of hydrogen in such materials and as to whether hydrogen is bound with oxygen (water) or another element such as a salt. At the same time, protons in other forms, such as hydrides, hydrates and acids, also are detectable and quantifiable.
The ability to make such determinations and the sensitivity of toroid-based NMR imaging makes this type of technique ideal for special nuclear material (SNM) characterization before such material is packaged. In fact, such nondestructive methods of examination of such SNM provides reductions in work force radiation exposure. Moreover, such techniques possibly could be used to determine select fission and activation products for a wide variety of critical applications (such as waste management and SNM management, control and accountability) and could be used to meet international safeguards with respect to the management of such products.
In one type of toroid cavity detector, a sample to be analyzed is placed in the toroid cavity and the toroid cavity is placed in an externally applied static main homogeneous magnetic field (B.sub.0). The presence of the B.sub.0 field causes the magnetic moments of a targeted class of nuclei in the sample to precess about the field's axis at a rate which is dependent on the magnetic field strength. Another magnetic field (B.sub.1) is produced that is perpendicular to the B.sub.0 field and is alternately energized and de-energized within the cavity. In the case where the B.sub.1 field is produced by a radio frequency (RF) transmitter pulse applied to an inner wire extending along the central axis of the cavity, the net sample magnetization is caused to rotate about the B.sub.1 field axis. Following the RF pulse, the response of the sample to the applied field is detected and in most applications, received signals from the energized nuclei serve as input signals for spectroscopic analysis of the sample.
One of the unique features of the toroid cavity detector compared to some other types of detectors is the full containment of the RF magnetic field flux that is generated when the inner wire is pulsed with an RF current. As a result, the RF magnetic field flux that emanates from a sample contained within the toroid cavity and subjected to an excitation pulse of electromagnetic radiation is completely captured (i.e., detected). This feature makes the toroid cavity detector two to four times more sensitive to weak nuclear resonance signals than conventional coil resonators and allows a quantitative measurement of the total number of NMR-active nuclear spins in the sample, not possible with currently available NMR detectors.
For samples placed in a magnetic field of sufficiently high homogeneity, it is possible to record different chemical species containing a common nucleus. Not only can a proton be distinguished from a uranium nucleus, but also protons in water molecules can be distinguished from protons in hydrogen gas molecules. In fact, protons in water molecules that are in different physical environments can be distinguished. The resolving capability of chemical shift makes the toroid cavity detector a useful device for analyzing and monitoring various types of radioactive materials from spent nuclear fuel to plutonium ash.
Another technique that can be used with a toroid cavity detector is rotating frame imaging (RFI). An asymmetrically shaped transmitter coil is used to achieve a B.sub.1 field gradient within the sample region. Different regions of the sample absorbs energy at different rates because the energy absorption process is controlled by the local strength of the B.sub.1 field. Thus, RFI can be used to image a homogeneously filled toroid cavity such that the relative quantity of sample that is located at each accessible value of the B.sub.1 field can be measured. As a result, this technique can be used to determine the diffusion coefficients of nuclei and the molecular species of the nuclei.
The toroid cavity detector also differs from other, more conventional electromagnetic detectors in that it produces a gradient in the generated RF magnetic field. This B.sub.1 field gradient has a mathematically well-defined spatial distribution. This magnetic field gradient feature has two significant consequences. First, the sensitivity of the toroid cavity is radially distributed, with the greatest sensitivity near the central axis of the device. This enables measurements on samples of limited quantity. Second, concentric annular regions of a sample contained in the toroid cavity exchange energy with the resonator circuit at different rates. Thus, analysis of energy transfer rates in a toroid cavity can yield a radial spatial mapping of the different nuclear constituents in a cylindrical sample container. The toroid cavity also has been proven to function with asymmetric conductors.
A simple extension of the spatial imaging capability of the toroid cavity detector is the ability to spin-label the sample with nuclear magnetization alternating in opposite polarities in a series of concentric cylindrical shells. With this labeling procedure, mobility of nuclei within the cavity can be measured. The mobility can be stochastic or coherent and it can be measured on a variable length scale extending from a few micrometers to several millimeters. This technique is suitable for measuring transport velocities of fluids and gases within a sealed container or at the interior surface of a canister where there might be perfusion.
As is disclosed in U.S. Pat. No. 5,574,370, the toroid cavity of a toroid cavity detector can be a cylindrically shaped, hollow housing which is closed at both opposite ends and made for example of copper. A central conductor consisting of an inner wire in a Teflon outer jacket is fed through the base of the cylinder and is positioned coincident with the major axis of the cylinder. In alternate embodiments disclosed in the '370 patent, slots are machined in the side walls, top and bottom of the toroidal cylinder to provide access to the toroid cavity for a fluid or gas sample being analyzed by the toroid cavity detector or a series of openings and sample holders can be positioned in the top of the toroidal cylinder in order to provide access for tubes containing the samples to be analyzed by the toroidal cavity detector.
In the toroid cavity detector of the type disclosed in the '370 patent, the B.sub.1 field is completely confined within the cavity formed in the housing of the toroid cavity detector and is generated when an RF signal is transmitted along the central conductor. The B.sub.1 field is strongest near the central conductor and drops off as the inverse of the distance toward the outer wall of the toroid cavity. Both sensitivity and distance resolution increase with the gradient in the B.sub.1 field and consequently, the toroid cavity detector provides a means to image simultaneously both the chemical shift of the targeted nuclei as well as their radial distance from the center and is well suited for NMR microscopy of films that uniformly surround the central conductor. In this regard, the amount of energy that is absorbed or transmitted by a sample within the toroid cavity detector varies with the location of the sample within the toroidal cavity such that multiple distances within the NMR active sample can be resolved by varying the transmitter pulse length. Because a homogeneous B.sub.0 field is used, the chemical shift information is not destroyed by the imaging process. In addition, such toroid cavity detectors appear to be useful for investigations of solids and polymers where broad lines usually limit the spatial resolution to 50-100 .mu.m. High-pressure and high-temperature capabilities of toroid cavities enable measurements of penetration rates in polymer or ceramic films in situ. In addition, electrochemical processes can be monitored when the central conductor is used as a working electrode.
The toroid cavity detectors of the type disclosed in the '370 patent have the advantage of providing a rugged reaction chamber that is readily machined from a variety of alloys, and the alternating magnetic field is highly confined within the cavity detector. Confining the alternating magnetic field minimizes sensitivity losses that occur through magnetic coupling with a high-pressure housing. Most importantly, the toroid cavity detector produces a well-defined magnetic field gradient, which, as noted above, varies with the inverse of the radial displacement from the center of the cavity. The resultant NMR intensity (I/I.sub.1) is predicted to depend on the transmitter pulse length, t, according to the following equation:
I/I.sub.0= 2.pi.h.intg.sin (.gamma.At/r)dr
where .gamma. is the gyromagnetic ratio, h is the height of the toroid, r is the radial distance from the center of the cavity, and A is the proportionality constant defining the magnetic field as follows: EQU B.sub.1 =A/r
The 1/r relation for the B.sub.1 field suggests that both the NMR sensitivity and the distance resolution should increase for materials that are close to the central conductor. Thus, the toroid cavity NMR resonator or detector is particularly powerful in the characterization of surface layers applied to the central conductor.
The use of a toroid cavity detector enables complete NMR spectral information to be retained during signal processing. Additionally, the strong gradient that is intrinsic to the torus enables the toroid cavity detector to provide a theoretical spatial resolution that is better than is possible with conventional MRI. Also, spatial resolution with the toroid cavity imager is less dependent on the line widths of the NMR signals used in the measurements because chemical shift information is not used to determine the distance as it is in conventional MRI.
Rotating Frame Imaging (RFI) uses the B.sub.1 field gradient of an asymmetrically shaped NMR transmitter coil to achieve its spatial resolution. The transmitted energy, and thus, the pulse rotation angle of the NMR active nuclei varies with the B.sub.1 field strength. Accordingly, a gradient can be used for spin localization. Through incremental increases in the pulse width, a set of amplitude-modulated spectra is derived in which the modulation frequencies yield the spatial information. Using a two-dimensional Fourier transformation, both the spatial and the chemical-shift information is resolved. This is a major advantage of the RFI technique over common MRI (Magnetic Resonance Imaging) where the chemical shift information is converted into a measure of distance by using a gradient in the main magnetic field B.sub.0.
While the toroid cavity detector of the type disclosed in the '370 patent provides these significant advantages, a phenomenon known as "probe ringing" can hinder any results that are obtained from such detectors. Due to the traditional configuration of a NMR solenoid probe, the application of a strong RF pulse to the coil induces eddy currents in nearby metallic structures which may be present as RF shields or as part of the magnet housing. In the presence of a static magnetic field, these eddy currents produce a mechanical torque in the metal. A mechanical shock wave results, and a reciprocal process generates an RF signal which is detected by the coil. This phenomenon is know as "probe ringing" and is often a problem when samples are being analyzed using low frequency pulsed RF signals. This so-called "coil disease" is also thought to occur in the metal of the solenoid coil itself--the DC magnetic field and the radio frequency currents that run perpendicular to the DC magnetic field produce a torque in the metal which causes acoustic waves that are reflected back and cause the ringing. The problem is that such "ringing" obscures the NMR peaks generated by the sample. After the RF pulse, the sample undergoes a free induction decay (FID) which lasts for several microseconds (for solids) to seconds (for liquids). Both the FID and probe ringing signals are coherent and temporally coincident. As a result, it is difficult to distinguish the NMR signals from the signals generated by the probe ringing phenomenon. In fact, the peaks of the signals from the sample will be masked if the magnitude of the FID signal is weaker than the probe ringing signal and the sample signal also will be difficult to extract if the duration of the FID is shorter than the duration of the acoustic ringing.
The problems that are caused by this probe ringing phenomenon also can mask the results being attained in a toroid cavity detector. The inner wire or central conductor extends through the bottom end of the cylindrical toroid cavity detector housing and along the central, elongated axis of the housing and is connected to the upper or top end of the housing. The top and bottom ends of the housing and the side walls of the cylindrical housing provide the necessary return path for the RF signal pulses. When the RF transmitted current is traveling along the central conductor of the toroid cavity detector, it is traveling parallel to the direction of the B.sub.0 field and it does not experience a force (torque). Thus, current that runs parallel to the direction of the B.sub.0 field does not cause probe ringing. However, the RF current then fans out at the inside of the top end of the housing toward the inside of the side walls of the housing. As the RF current travels along the inside of the top end of the housing and along the inside of the bottom end of the housing, a mechanical force (torque) is generated that is simultaneously perpendicular to the B.sub.1 and B.sub.0 fields. This results in the subsequent deleterious probe ringing phenomenon.
In the case of the toroid cavity detector disclosed in the '370 patent, this probe ringing phenomenon is magnified because the top and bottom end portions of the toroid cavity detector housing are in the intense and homogeneous portion of the B.sub.0 field. Moreover, the material which the housing is made, including in particular the top and bottom end portions, does not tend to adsorb much of the mechanical torquing that is caused by the flow of the RF current pulses. Instead, the housing material disclosed in the '370 patent is very effective at converting RF current torque into a mechanical impulse. By a reciprocal process, mechanical shock waves resulting from the mechanical impulse (that in turn was caused by the RF current torque) are reflected from metal/air interfaces and are efficiently back-converted to RF current. The back converted RF current is coherent and, therefore, indistinguishable from NMR signals.
Accordingly, it is an object of the present invention to provide a new and improved toroid cavity detector used in NMR tomographic imaging of a sample, in which the effects of probe ringing is greatly reduced.
It is another object of the present invention to provide a new and improved toroid cavity detector having a cylindrical housing that is elongated sufficiently in length so that the top and bottom portions, where the significant parts of the effect of probe ringing is generated, are in the weaker, fringe areas of the static main magnetic field (B.sub.0) in which the housing is placed.
It is still another object of the present invention to provide a new and improved toroid cavity detector wherein certain types of material are provided along at least the inside or outside of the top and bottom ends of the cylindrically shaped toroid cavity to adsorb RF current induced mechanical shock waves that cause the effect of probe ringing.
It is yet another object of the present invention to provide a new and improved toroid cavity detector having a cylindrically shaped toroid cavity that is elongated sufficiently in length so that the top and bottom portions of the cavity are located in the weaker, fringe areas of the static main magnetic field (B.sub.0) but spacers are provided adjacent to the internal top and bottom portions of the toroid cavity so as to maintain the sample being analyzed in the more intense and homogeneous portions of the B.sub.0 field.